1. Field of the Invention
The invention relates to a lens-fitted film unit with a taking lens comprising injection molded plastic lens elements.
2. Description of Related Art
Lens-fitted film units having been on the market are inexpensive and make one easily enjoy taking pictures. Such a lens-fitted film unit having a case in which a simple exposure mechanism is installed and a roll of film is pre-loaded. The lens-fitted film unit is widely spread because of the convenience that it can be gotten whenever and anywhere one wishes to take pictures and is put to a photo-shop as it is for processing and printing. There are variety of lens-fitted film units ranging from the most simplified unit which incorporates a fundamental exposure mechanism only to the most sophisticated unit which is equipped with a taking lens suitable for telephotography and/or a strobe unit for night photography and indoor photography. These lens-fitted film units give one a wide choice according to one's intentions.
One of advantages of the lens-fitted film units is low production costs. Since it is mandatory to provide the lens-fitted film units at as low costs as possible, such a lens-fitted film unit is under the necessity of having a quite simplified structure. For example, the taking lens is held in position by a lens holder formed with a fixed aperture so as to focus images of standard sharpness of as many objects as possible. The taking lens comprises one or two lens elements. The taking lenses have focal lengths from about 30 mm to 35 mm for lens-fitted film units with a 35 mm format film pre-loaded therein and a focal length of about 24 mm for lens-fitted film units with a 24 mm format film pre-loaded therein. Further, for lens-fitted film units suitable for telephotograph, the taking lenses have focal lengths increased to about 100 mm.
Typically, plastic lenses formed by injection molding are employed as component lens elements of the taking lens. While such an injection molded plastic lens is suitable to be mass-produced and produced at production costs lowered as compared with glass lens elements, the injection molded plastic lens has the drawback that it easily varies its refractive power due to a change in ambient temperature. In particular, the injection molded plastic lens expands to weaken its curvature with the result of becoming more apt to decrease the refractive power as the ambient temperature rises. If the refractive power of the taking lens becomes too small, the taking lens shifts its focal point behind a film strip situated in a predetermined film plane, forming a blurry image on the film strip.
Therefore, the two component taking lens for the lens-fitted film unit is typically comprised of two plastic lens elements having different thermal expansion coefficients so as to control a shift of the focal point of the taking lens due to a change in ambient temperature. The longer the focal length of the taking lens is, the grater a change in refraction power of taking lens caused due to a change in ambient temperature becomes, and when the taking lens has a focal length longer than 50 mm, it is difficult to control a change in refractive power of the taking lens only by using different materials for the two plastic lens elements. In particular, when the taking lens suitable for telephotograph has a focal length of approximately 100 mm, it is completely impossible to control a change in refractive power of the taking lens only by using different materials for the two plastic lens elements.